For forming a laminated coating film by a coating method in which two kinds of coating materials are applied using a wet-on-wet technique and then baked, there has been a conventionally used method by which the laminated coating film as a whole is cured. In this method, thermosetting coating materials for forming layers constituting a laminated coating film are selected so that all the layers can be cured at the same heating temperature after all the coating materials are applied. However, the conventional coating method has a problem that the obtained laminated coating film is inferior in surface texture and gloss to that obtained by baking a lower layer and then applying and baking a coating material for forming an upper layer. In this connection, various methods have been proposed to improve the surface texture and the gloss of a laminated coating film.
For example, Japanese Unexamined Patent Application Publication No. 2007-283271 (PTL 1) discloses a method for forming a multilayer coating film, the method comprising: forming a base coat film on a workpiece by applying a water-based colored base coating material containing an amino resin such as melamine as a cross-linking agent; applying and stacking a water-based clear coating material containing a polyisocyanate compound as a cross-linking agent on the base coat film remaining in an uncured state using a wet-on-wet technique; and subsequently curing the base coat film and the clear coat film together by heating, wherein the solid content concentration and the water absorption percentage of the base coat film at the application of the clear coating material are set within certain ranges, i.e., the solid content concentration of the base coat film is 85% by mass or higher, and the water absorption percentage of the base coat film at 20° C. is 10% by mass or less. PTL 1 also discloses an article coated by the method for forming a multilayer coating film. However, in the case of the conventional method for forming a multilayer coating film and the article coated by the method as described in PTL 1, the appearance qualities, such as surface texture (smoothness) and gloss, of the laminated coating film are not necessarily sufficient, and it is difficult to improve the surface texture and gloss to the levels required for the appearance qualities of automobiles. In this respect, coated articles having better appearance qualities and better durability have been demanded for automobile steel plates and the like, and further improvement of the wet-on-wet coating method has been desired.